The present invention relates to spot-type disc brakes in general, and more particularly to a disc brake with additional axial force application to a region of the brake shoe.
There is already known a spot-type disc brake construction which includes at least one force-actuated brake shoe which is supported in a circumferential direction, at least at the disc exit edge as considered in the forward direction, on a brake shoe carrier member via a braking-force bell crank lever pivotable about a substantially radial tilting axis. In this construction, a first lever arm of the lever abuts on the disc exit edge of the backing plate of the brake shoe in such a manner that it receives frictional forces acting in the circumferential direction on the brake shoe. However, the first lever arm permits axial sliding movement between the backing plate and the first lever arm. The tilting axis of the bell crank lever is supported in the circumferential direction on the brake shoe carrier member. The second lever arm of the lever extends to the back of the backing plate of the brake shoe and exerts on the latter an additional axial force which depends upon the braking force and the ratio of the lengths of the lever arm. Usually, the brake shoe carrier member is constituted by a brake carrier element. However, in a floating-caliper brake, the floating caliper is also able to support one brake shoe or both of them in the circumferential direction.
It is known that the piston-close brake lining in disc brakes and ring disc brakes is subjected to greater wear on the disc entry edge than on the disc exit edge. This is attributable both to the friction forces at the lining supporting area and to the displaced positioning of the supporting area relative to the friction surface, which results in an inclinedly acting torque. This effect is particularly disadvantageous in caliper brakes with relatively wide brake shoes.
To counteract the one-sided increased wear of the brake lining, it has already been proposed to displace the piston in the circumferential direction off-center towards the disc exit edge. However, this solution has the disadvantage that the brake and, consequently, its deformation become asymmetrical.
A spot-type disc brake has already been proposed in the British Patent Specificiation No. 829,866, which is capable of augmenting the braking force. In this construction, braking-force bell crank levers are provided on both the disc entry and the disc exist edges, these levers being mounted on the brake support member for pivoting about substantially radially extending shafts. The first lever arm of each of these braking-force bell crank levers abuts on the disc entry or on the disc exit edge at the backing plate of the brake shoe in such a manner that it receives frictional forces acting in the circumferential direction on the brake shoe and transmits them to the second lever arm which rests against the back of the backing plate. When braking, the brake shoe is entrained in the circumferential direction by the brake disc and acts on one of the two first lever arms of the braking-force bell crank levers. This results in the application of an additional braking force to the brake shoe in the axial direction, with attendant augmentation of the primary braking force which was initially applied from the outside.
However, the disadvantage of this known spot-type disc brake is that, with increasing wear of the brake linings, the braking-force bell crank levers have to be pivoted by a gradually increasing amount to retain the second lever arms in abutment with the carrier plate. As a result of this, the first lever arm gradually moves further away from its abutment surface on the backing plate of the brake shoe.